I. Field of the Invention
The present invention relates generally to methods and apparatus for testing semiconductor devices using electron emission spectrometry. More particularly, the present invention relates to using a time of flight electron detector as a device to differentiate electrons based on their energy. This can be utilized to determine the composition of an area of interest on a semiconductor device.
II. Background
The industry of semiconductor manufacturing involves highly complex techniques for integrating circuits into semiconductor materials. Due to the large scale of circuit integration and the decreasing size of semiconductor devices, the semiconductor manufacturing process is prone to processing defects. Testing procedures are therefore critical to maintain quality control. Since the testing procedures are an integral and significant part of the manufacturing process, the semiconductor industry constantly seeks more accurate and efficient testing procedures.
A critical aspect of semiconductor fabrication involves the formation of the multiple conductive layers and liner layers. Each conductive layer includes the metal traces, also referred to as interconnects, which form the paths along which electronic signals travel within semiconductor devices. Dielectric material layers and liner layers separate conductive layers. The dielectric material layer, commonly silicon dioxide, provides electrical insulation between the conductive layers. Portions of each conductive layer are connected to portions of other conductive layers by electrical pathways called “plugs.” The liner layers are formed between each conductive layer and each dielectric material layer to prevent the conductive material from diffusing into the dielectric material layer. The liner layer inhibits a conductive layer from diffusing into an underlying dielectric and shorting circuiting with an adjacent conductive layer. Of course, such short circuit formations are likely to be detrimental to semiconductor performance. In particular, copper, a common conductive material used in semiconductor devices, diffuses very aggressively into silicon dioxide. The thickness and composition of the conductive and liner layers must be formed under extremely small margins of error. Thus, systems capable of testing the characteristics of these layers are very important.
Typically, testing of semiconductor devices is performed in two phases. The first phase involves an inspection process and the second phase involves defect review. During the inspection process, potential defects in semiconductor devices are typically detected through die-to-die comparison techniques. This is where similar die areas are compared against each other and differences between die areas are noted as possible defects since the die areas are ideally identical to each other. Due to the relatively low resolution of inspection systems, the actual nature of these potential defects cannot be determined. Therefore, semiconductor devices with potential defects are identified and set aside for the second phase of defect review. During defect review, these potentially defective semiconductor devices are studied more carefully to determine the nature and/or cause of these potential defects.
Some of the techniques that have been used during defect review include Energy Dispersive X-ray Spectroscopy (EDX) and Energy Dispersive Spectrometry (EDS), which can each be used to determine the material composition of an area of interest on a semiconductor device, such as a defect. Specifically, EDX involves exposing the area of interest to a beam of charged particles, which causes the area of interest to emit x-rays characteristic of the materials at the area of interest. A detector is then used to collect a portion of the x-rays to determine the energy spectrum for the collected x-rays, which can be used to identify the materials. The detector is typically positioned adjacent to the area of interest and the beam of charged particles, such that the detector does not interfere with either the area of interest or beam of charged particles. The detector is also positioned below the objective lens, which focuses the charged particle beam onto the area of interest, of the inspection system. However, because of the close proximity between the detector and the area of interest, only a portion of the x-rays can be collected, thereby limiting the amount of information that can be obtained about the area of interest. Specifically, only the x-rays emitted in the direction of the detector will be measured even though x-rays typically are emitted in multiple directions. In order to collect a broader range of the emitted x-rays, either multiple detectors or a single detector repositioned at various locations must be used, both of which can be time-consuming and costly. Even if multiple detectors or a mobile detector is used, many of the x-rays will go undetected, especially those emitted in the direction of the oncoming beam of charged particles. Furthermore, because materials with low atomic numbers disperse few x-rays, the EDX technique does not work well for analyzing some materials.
The EDS technique, as described in more detail below, also involves exposing the area of interest to a beam of charged particles. However, instead of analyzing x-rays emitted by the area of interest, EDS analyzes electrons emitted by the area of interest in response to being exposed to the beam of charged particles. In particular, EDS typically involves applying an electromagnetic field to the emitted electrons, such that the electrons are separated spatially depending on their respective energies. Next, each spatial region of interest is monitored to determine the number of electrons passing through these regions. Based on the number of electrons passing through various spatial regions, an energy spectrum can be obtained for the collected electrons. Because each spatial region is typically analyzed in sequence, EDS can be a time-consuming process. For instance, generating a spectrum for a single area of interest using EDS can take about ten minutes.
Accordingly, conventional systems for obtaining an energy spectrum and determining the composition of an area of interest on a semiconductor device are time-consuming, and can be of limited value, especially when only a portion of the spectrum can be obtained. In view of the foregoing, there are continuing efforts to provide improved methods and apparatus for testing and reviewing semiconductor devices.